The present invention relates generally to the production of radioisotopes and more specifically to a target comprising of a target body and a material sample confined in the target body to be irradiated by a beam of charged particles for producing a radioisotope.
A radioisotope may be produced based on various nuclear reactions by irradiating a material sample with a particle beam produced in an accelerator. A typical medical application is Positron Emission Tomography (PET). The nuclear medicine PET procedure is used for imaging and measuring physiologic processes within the human body. A radiopharmaceutical is labeled with a radioactive isotope and is suitably administered to a patient. The radioisotope decays inside the patient through the emission of positrons. The positrons are annihilated upon encountering electrons which produce oppositely directed gamma rays. A PET scanner includes detectors surrounding the patient which detect the paths of gamma rays. This data is suitably analyzed to map the presence of the radioisotopes in the patient for diagnostic purposes.
The commonly used radioisotopes for PET procedure are Fluorine-18 (18F), Oxygen-15 (15O), Nitrogen-13 (13N) and Carbone-11 (11C). The most common method of producing these isotopes is by irradiating their respected material samples by a beam of energetic proton. The material sample during the irradiation is confined in a target which comprises of a cavity for holding the sample and a thin foil at the entrance of the cavity to confine the sample. The irradiating beam passes through the thin foil and reaches the material sample. With a beam of proton the material samples to be irradiated are Oxygen-18 water for production of Fluorine-18, Nitrogen-15 gas for production of Oxygen-15, Oxygen-16 water for production of Nitrogen-13, and Nitrogen-14 gas for production of Carbon-11. The irradiating proton energy ranges from about 9 MeV to up to 18 MeV. Production of the above four radioisotopes requires four designated targets, one target for each radioisotope. The targets that uses Oxygen-18 and Oxygen-16 water as material samples are commonly referred to as the water targets and the other two that uses gas as material samples are referred to as the gas targets.
It is highly desirable to produce all four PET isotopes in a single target. This reduces the cost of building and maintaining four targets to one single target. One of the object of the prevent invention is to produce all four PET isotopes in one single target.
Fluorine-18 is the most widely used radioisotope for PET application. It has a half-life of less than two hours. Accordingly, the radioisotope must be produced daily before being administered to the patient. The material sample of this radioisotope, Oxygen-18 water, is very expensive and there is also a shortage of Oxygen-18 water world wide. Accordingly, it is desired to produce a large quantity of this radioisotope with as little of Oxygen-18 water as possible. This could be achieved by increasing the proton beam current bombarding the target. However, as the proton beam current increases the existing water targets suffer from many undesirable problems. These problems stem from the poor heat conductivity of water which cannot transfer the absorbed heat from the beam to the target body. Boiling, which is the reaction of water to excess heat and a means of transferring heat to the target body causes bubbles or the so called voids to be formed along the beam path. Subsequently the target becomes thin if the target depth, which is by definition the length of the water column as seen by the beam, is not already overcompensated. In a thin target the beam strikes the back of the target before losing its energy in water. The results are poor yield in addition to harmful sputtering of the target body material in the water which can be followed with unwanted nuclear reactions with beam and stable chemical reactions with fluoride ions. For production of Curie level of Fluorine-18 the costly method of dealing with the above problems has been to increase the depth of the target up to ten times the proton range in water. A target with this large depth defeats the primary consideration in design which is to consume as little of the expensive Oxygen-18 water as possible.
Accordingly, it is highly desired to provide a target which is configured for high proton beam current that can also use very little of expensive Oxygen-18 water for production of Curie levels of 18F radioisotope. The other object of the present invention is to reduce the consumption of Oxygen-18 water for production of a given amount of Flourine-18 to about one tenth of a conventional water target. Additional object of the present invention is to eliminate all the noted problems of a water target. Further object of the present invention is to make the target suitable for accepting a high power beam.
Furthermore, it is well known that all gas targets develop density depression when irradiated with a moderate or high power beam. The density depression develops in the interaction volume—the volume that the beam interacts with the sample to produce an isotope. The density depression causes poor yield and also causes the beam to strike the back of the target body. Moreover, because of the density depression the target can become unstable. The problems noted here are not limited to a gas target. They should also occur in a steam target.
Accordingly, it is also highly desirable to prevent the density depression in a gas and a stream target and to suppress other instabilities which can develop in the target as the beam power increases. It is further the object of the present invention to prevent the density depression in gas and steam targets and suppress other instabilities that can develop as the beam power increases.